Thread: fm modulator: high pitch whine

I'm using the FM modulator as seen here:
http://www.davidnavone.com/linedrivers.html
N-724

there is a high pitch "whine" that increases when I turn the volume on the radio up. It's there when the car is on or off. it's there even when there are no inputs plugged into the modulator.

I contacted the manufacturer, and they suggested that it was alternator whine. I didn't think there would be alternator whine when the car is off ...
they suggested "filtering" the input current to the modulator. How can I filter the power coming into it? a voltage regulator of some sort?

Thanks
-Matt